1. Field of the Invention
The present invention relates to a horizontal, single chamber front loading vacuum heat treating furnace capable of rapidly cooling the heat treated materials by insitu gas quenching at pressures up to 20 Bar and gas velocities approaching 200 miles/hour in a single chamber of the furnace.
2. Description of the Prior Art
The need for “green” environmentally friendly quenching capabilities has challenged furnace engineers to design furnaces capable of achieving high pressure, rapid gas quenching up to 20 Bar or greater. The ability to achieve such high gas pressures for cooling is driven by the need to eliminate the use of oil quenching for cooling of tool steels. Oil quenching is currently the fastest quenching medium for heat treatment of tool steels. However, it is rife with disadvantages, such as cracking and distortion due to high residual stress on the processed material. Some tool steel grades with reasonable cross-sections have been successfully quenched in a 10 Bar vacuum furnace, but there are limitations of the load size and part dimensions which need to be overcome. The use of 10 Bar quenching for lower alloy tool steels is limited. See, for example, U.S. Pat. No. 7,514,035. The present inability to gas quench such alloys led to the design and development of the present invention, which can achieve cooling gas pressures as high as 20 Bar and gas velocities approaching 200 miles/hour for rapid cooling of high and low alloy steels.
Typical high pressure gas quench multi-bar furnaces utilize a standard design of fixed thermal exit baffles and complicated fan cycling sequences—clockwise and counterclockwise—with directional control of the wind mass in order to achieve the necessary velocity within the furnace chamber in order to achieve the high pressures necessary to cool tool steels. See, for example, U.S. Pat. No. 5,478,985. Typical gas flow designs have stationary exit ports, which result in restricted gas flow and unacceptable pressure drops.
The use of movable baffles or bungs has been utilized previously in vacuum heat treat furnaces (see U.S. Pat. Nos. 6,903,306 and 8,088,328). However, the movable dampers in U.S. Pat. No. 6,903,306 are connected to a rigid fixed-size opening that is connected to one side of a narrow plenum. The bungs from U.S. Pat. No. 8,088,328 have a large plenumless gas chamber; however the gas must flow through a gas duct in order to pass through an external heat exchanger. These passageways result in high pressure drops, which are fine for an atmosphere quench design, but detrimental for quench pressures up to 20 Bar. The present design eliminates these limitations and therefore does not suffer from the gas flow restrictions and pressure drops of prior art designs.
Prior art references—U.S. Pat. Nos. 4,836,776, 4,906,182 and 5,478,985—teach improved gas flow, but the fixed gas baffle designs in these references suffer from high pressure drops leading to decreased energy transfer and longer cooling times and slower cooling rates. The configuration in U.S. Pat. No. 4,836,776 includes a movable baffle which feeds the hot gas from the furnace toward the heat exchanger and fan in a lateral direction, resulting in a higher pressure drop. The use of bearing assemblies in these baffle designs will eventually lead to mechanical failure, resulting in increased maintenance downtime and loss of service time for the furnace. These designs also suffer from direct flow of hot gases from the furnace into the center of a water-cooled heat exchanger and recirculation fan. Although the teaching in U.S. Pat. No. 7,514,035—that the fixed baffle design with gas recirculation systems and mammoth ducts helps prevent pressure drops—this design has its limitations, as much higher gas pressures (greater than 10 Bar) are utilized. In order to achieve cooling gas pressures up to 20 Bar without loss of cooling gas velocity due to high pressure drops, a new design for flow of hot gas to the heat exchanger and recirculation fan was required. The present design is an improvement over the teachings in U.S. Pat. No. 7,514,035 and in the other prior art references mentioned above.
The design and teachings of the present invention result in the ability to achieve high pressure quenching up to 20 Bar with gas cooling velocities approaching 200 miles/hour, thereby rapidly cooling the work piece by diffusing the gas through a much larger opening directly into the heat exchanger and the blower fan and returning the cooled gas into a large gas chamber insitu. Accordingly, pressure drops associated with typical gas quench furnaces have been eliminated. A key feature of the present design is the movable doors which provide a dual purpose. When the doors are closed during the heating cycle, they provide a mechanism for retaining temperature uniformity during the heating cycle throughout the hot zone, and they serve to prevent thermal radiation from leaking around the baffles, such as in the stationary baffle designs of the prior art.
Similar techniques have been utilized in prior art designs, such as in U.S. Pat. No. 8,088,328, which discloses an atmosphere furnace that is not designed for high pressure gas quenching. The present design has the opening at the rearward end of the furnace directly attached to a heat exchanger with a more compact design that incorporates a high surface area per unit volume. Another key feature of the present invention is the coupling of this large opening with a specially designed chevron-type baffle made of stainless steel. The chevron baffle is situated behind the movable radiation shield doors and is not exposed to the heat from the heating cycle until the gas quenching cycle is initiated. The purpose of the chevron baffle is two-fold. One purpose is to serve as a gas diffuser, and the other is to serve as a radiation barrier of the radiant heat from the furnace hot zone to the heat exchanger at the beginning of the quenching cycle. The heat exchanger will only be exposed to convective heat from the recirculation of the cooling gases.